This invention relates to a method of dredging up sludge, which is deposited on the bottom of the water, continuously in a high density and sending the resultant sludge under pressure to a predetermined place, and an apparatus used to practice this method.
In order to remove the sludge deposited on the bottom of the water in, for example, a dam and a harbor, a method of sending up the sludge with the water by using a dredging pump is employed. However, according to this dredging method in which a large quantity of water is sent up with the sludge, large power and equipment are required.
Moreover, in order to separate sludge from the muddy water sent up by a dredging pump, it is necessary to secure a vast land to be reclaimed from the sea. It is difficult to carry out a dredging operation in a place where such a vast land to be reclaimed from the sea is not available. Since the separating of sludge from the muddy water is generally done in a small land to be reclaimed from the sea, a lot of time is required to carry out the operation.
In order to dredge up the sludge efficiently by using small power and a small land to be reclaimed from the sea, it is necessary that the sludge be brought up from the bottom of the water continuously in a high density. To meet this requirement, it is desirable to send up the sludge with the shape of the layer thereof kept as it originally was while preventing to as great an extent as possible the water from mixing in the sludge. In a dredging operation carried out with a view to achieving this purpose, various types of sludge sucking devices which suit the purpose are used. For example, suction devices based on a centrifugal pump system and a pneumatic pump system have been proposed.
In these systems, the sludge deposited on the bottom of the water is sent up by sucking the sludge as it is into the pump, or agitating the sludge to increase the fluidity thereof and sucking the resultant sludge, which is akin to muddy water, into the pump. Accordingly, the sucked sludge of an increased fluidity flows into the suction port along the flow of the muddy water. In case of sucked sludge having a fluidity close to that of water, a channel of water is formed early in the sludge in most cases except the case where the sludge has, for example, a water content of not lower than 200% (namely, a content of water two times as large as the weight of dry sludge). Therefore, a large quantity of water is sucked with sludge, i.e., water the quantity of which is far larger than that of the sludge is necessarily sent up, so that the dredging efficiency greatly decreases.
As may be inferred from such phenomena, it is very difficult to dredge up sludge continuously in a high density by utilizing these suction devices and a flow of the muddy water, due to the principle that the water flows more smoothly than sludge. This makes it necessary to use a large suction device and large power.
A dredging apparatus in which the air is supplied to the upper portion of a casing, which is opened horizontally and provided with a screw horizontally therein, so as to discharge the water has also been developed. However, in this apparatus, water with sludge is fed from the front portion of a casing. Therefore, it is impossible to send up sludge with a high efficiency as in the above-described apparatus made according to the known techniques.